Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals or channels, each having a different wavelength, are combined onto an optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the optical signal output from the laser, and an optical combiner to combine each of the modulated optical signals. The wavelengths are typically separated from one another by a channel or spectral spacing.
Typically, the optical signals are modulated in accordance with a modulation format. Various modulation formats are known, such as on-off-keying (OOK), differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK), binary phase shift keying (BPSK). As generally understood, different modulation formats may have different optical characteristics. For example, certain modulation formats may be more sensitive to noise, and thus may be associated with a higher bit error rate if noise is present on a given optical link. In addition, some modulation formats may have a higher spectral density and thus can carry more data per unit of spectrum than others. Still others may have a higher tolerance for polarization mode dispersion (PMD), such that certain modulation formats may require little or no PMD compensation compared to others for a given amount of PMD.
In general, those modulation formats that have a higher spectral density, such that more information or bits are carried per unit of spectrum, will typically have less energy per bit. As a result, high spectral density modulation formats are more susceptible to transmission non-idealities, and thus will have higher bit error rates for a given amount of PMD or optical signal noise, for example. Accordingly, such modulation formats may be used to carry data at relatively higher rates over shorter distances. On the other hand, those modulation formats that require more energy per bit may have lower bit error rates and are spectrally less efficient. Such low spectral density modulation formats, therefore, may be used to carry data over longer distances.
Conventional WDM systems typically include a series of printed circuit boards or cards, such that each one supplies or outputs a corresponding optical channel. Such cards typically include discrete components, such as a laser, modulator, and modulator driver circuit which are associated with each channel. Typically, different cards are provided for different optical links, such that optical signals having an appropriate modulation format are supplied to a given link. For example, specific cards may be provided to supply signals that are transmitted over long distance links, such as those which may be used in undersea or submarine systems, while other cards may be provided to supply signal to shorter distance terrestrial links. Thus, cards are often tailored for different optical links. As a result, the costs for manufacturing each card may be excessive.
Moreover, fiber optic communications systems for transmitting with a spectral efficiency 2 bits/s/Hz typically may use a PM-QPSK (polarization multiplexed-quadrature phase shift keying) modulation format. Although this modulation format performs well for links up to about 2000 km, beyond that, PM-QPSK signals may have a relatively high number of errors (i.e., have a high bit error rate) that typically cannot be corrected with conventional forward error correction (FEC) techniques. Accordingly, there is a need for a WDM transmitter that can transmit optical signals having a modulation format that has lower spectral efficiency for transmission over longer distances or over optical links having significant impairments (e.g. noise or non-linearities, such as cross-phase modulation or four wave mixing) and can also transmit optical signals having another modulation format that can transmit over shorter distances or over links have reduced impairments. In other words, there is a need for a WDM system that has optimized data carrying capacity